1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus, and particularly to a discharge lamp lighting apparatus for lighting a plurality of discharge lamps used as a backlight for a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (LCD) device is extensively used as a display device of an electronic device, and the like, and is increasingly replacing a cathode ray tube (CRT) in a personal computer or a television with a relatively large display. In an LCD device for use with such a large display, a backlight device to light a plurality of discharge lamps is used in order to achieve a sufficient display brightness in a uniform manner.
A discharge lamp is usually lit by a discharge lamp lighting apparatus including an inverter. In order to prevent smoking and firing at the time of abnormal output, such an inverter is often provided with a sense circuit to detect an abnormal lamp current flowing in a discharge lamp in the case of a short circuit or an open circuit at its output, and also with a protection circuit to perform an operation to protect circuit elements according to an output signal from the sense circuit. In a conventional backlight device for a plurality of discharge lamps as described above, a malfunction detecting circuit is provided for each of the plurality of discharge lamp in order to reliably detect problems with lamp currents.
A discharge lamp of an increased length is increasingly employed, especially, in a backlight device for a large LCD device described above, and a discharge lamp lighting apparatus to light such a discharge lamp with an increased length is adapted to light the discharge lamp such that an opposite phase voltage outputted from an inverter connected to the discharge lamp is applied to the both electrodes of the discharge lamp. In such a discharge lamp lighting apparatus, a malfunction detecting circuit for lamp currents is preferably provided at the both sides of each of the discharge lamp thus requiring the malfunction detecting circuit in a number twice as many as that of the discharge lamps.
FIG. 5 is a circuit diagram of a conventional discharge lamp lighting apparatus as described above. Referring to FIG. 5, a discharge lamp lighting apparatus 100 is adapted to light a plurality (2n pieces: n is an integer equal to one or larger) of U-shape cold cathode lamps La1 to La2n in a controlled manner, and includes a plurality of lighting circuit blocks LC1 to LCn each having a pair of discharge lamps (La1+La2)/(La3+La4)/ . . . /(La2n−1+La2n) connected thereto, a plurality of bridge circuits BR1 to BRn provided respectively at the lighting circuit blocks LC1 to LCn, and a control circuit 11 to drive the bridge circuits BR1 to BRn in a controlled manner.
The lighting circuit block LC1 includes stet-up transformers T1 and T2 (step-up transformer group), and sense circuits SC1 to SC4. The step-up transformer T1 is a two-output transformer which includes two primary windings Np11 and Np12 connected in series to each other, and two secondary windings Ns11 and Ns12 independent of each other, and also the step-up transformer T2 is a two-output transformer which includes two primary winding Np21 and Np22 connected in series to each other, and two secondary windings Ns21 and Ns22 independent of each other. In the step-up transformer T1, the secondary winding Ns11 has its high voltage side output connected to one electrode of the discharge lamp La1 and its low voltage side output connected to the sense circuit SC1, and the secondary winding NS12 has its high voltage side output connected to the other electrode of the discharge lamp La1 and its low voltage side output connected to the sense circuit SC2. Likewise, in the step-up transformer T2, the secondary winding Ns21 has its high voltage side output connected to one electrode of the discharge lamp La2 and its low voltage side output connected to the sense circuit SC3, and the secondary winding NS22 has its high voltage side output connected to the other electrode of the discharge lamp La2 and its low voltage side output connected to the sense circuit SC4.
The sense circuit SC1 includes a sense resistor Ra1, diodes Da1 and Dc1, and comparators CPo1 and CPs1. The sense resistor Ra1 has its one terminal connected to the low voltage side output of the secondary winding Ns11 of the step-up transformer T1 and its other terminal connected to ground. The anodes of the diodes Da1 and Dc1 are connected to the connection point between the low voltage side output of the secondary winding Ns11 and the sense resistor Ra1. The output from the cathode of the diode Da1 is applied to the positive input terminal (+) of the comparator CPo1 and to the negative input terminal (−) of the comparator CPs1, and predetermined reference voltages Vth1 and Vth2 are applied respectively to the negative input terminal (−) of the comparator CPo1 and the positive input terminal (+) of the comparator CPs1. And, the anode of a diode is connected to the output terminal of each of the comparators CPo1 and CPs1, and the cathodes of respective diodes are connected to each other.
The other sense circuits SC2, SC3 and SC4 are structured identically with the sense circuit SC1, and are connected to the low voltage side outputs of the respective secondary windings Ns12, Ns21 and Ns22 in the same way as the sense circuit SC1. Also, the lighting circuit blocks LC2 to LCn are structured identically with the lighting circuit block LC1.
In the lighting circuit blocks LC1 to LCn structured as described above, all the cathodes of the diodes Dc1 to Dc4n are connected to one common line which is connected to ground via a sense resistor Rc1. The connection point between the sense resistor Rc1 and the common line is connected to an error amplifier 22 of the control circuit 11. On the other hand, all the cathodes of the diodes connected to the output terminals of the comparators CPo1 to CPo4n and the comparators CPs1 to CPs4n are connected to another common line that is different from the common line connected to the diodes Dc1 to Dc4n, and that is connected to a protection circuit 25 of the control circuit 11.
The bridge circuit BR1 connected to the lighting circuit block LC1 is a full-bridge circuit structured such that a series circuit composed of switch elements Q1 and Q3 and connected across a DC power supply Vin is connected in parallel to a series circuit composed of switch elements Q2 and Q4, and the series connected primary windings Np11+Np12 of the step-up transformer T1 and the series connected primary windings Np21+Np22 of the step-up transformer T2 are connected in parallel to each other between the connection point of the switch elements Q1 and Q3 and the connection point of the switch elements Q2 and Q4.
Also, the bridge circuits BR2 to BRn which are identical with the bridge circuit BR1 are connected respectively to the lighting circuit blocks LC2 to LCn in the same way, and common gate driving signals d1 to d4 sent from the control circuit 11 are supplied respectively to the switch elements Q1 to Q4 of each of the bridge circuits BR1 to BRn.
Description will now be made on a normal time lighting operation of the discharge lamp lighting apparatus 100. The control circuit 11 mainly includes an oscillation circuit 21 as a CR oscillation circuit, the aforementioned error amplifier 22, a PWM circuit 23, a logic circuit 24, and the aforementioned protection circuit 25. The oscillation circuit 21 generates a triangular wave 21a at a predetermined frequency corresponding to the values of an external resistor 26 and an external capacitor 27 and sends to the PWM circuit 23. The error amplifier 22 compares between the voltage of a feedback signal 22b and a predetermined reference voltage Vref and supplies the PWM circuit 23 with an output 22a having a voltage corresponding to the difference therebetween. The PWM 23 compares the triangular wave 21a and the output 22 of the error amplifier 22, generates a predetermined PWM pulse 23a and sends to the logic circuit 24. The logic circuit 24 generates appropriate gate driving signals d1 to d4 according to the triangular wave 21b sent from the oscillation circuit 21 so as to alternately switch on and off two pairs of switch elements, one of which is composed of the switch elements Q1 and Q4, and the other of which is composed of the switch elements Q2 and Q3, whereby the bridge circuits BR1 to BRn are driven.
Thus, in the discharge lamp lighting apparatus 100, an AC voltage with a predetermined frequency is generated at the primary side of each of the step-up transformers T1 to T2n, and the AC voltage generated is boosted by the step-up transformers T1 to T2n, wherein two outputs of each of the step-up transformers T1 to T2n are arranged such that voltages having their respective phases reversed from each other are applied to both electrodes of the discharge lamps La1 to La2n which are connected to the respective high voltage sides of the two outputs, whereby the discharge lamps La1 to La2n are efficiently lit in a controlled manner.
During this lamp lighting operation, the lamp currents flowing in the discharge lamps La1 to La2n are rectified by the diodes Dc1 to Dc4n of the sense circuits SC1 to SC4n, and the maximum current of the lamp current flowing in each of the discharge lamps La1 to La2n is converted into the feedback signal (voltage) 22b by the sense resistor Rc1, and the feedback signal (voltage) 22b is inputted to the error amplifier 22 of the control circuit 11.
The control circuit 11 controls the switching operation of the bridge circuits BR1 to BRn by the PWM method according to the feedback signal (voltage) 22b thereby regulating the electric power supplied to the step-up transformers T1 to T2n, which enables control of the lamp currents of the plurality of discharge lamps La1 to La2n. 
Further, in the sense circuits SC1 to SC4n, the sense voltages generated at the sense resistors Ra1 to Ra4 are rectified by the respective diodes Da1 to Da4n and inputted to the comparators CPo1 to CP04n and the comparators CPs1 to CPs4n, whereby the sense circuits SC1 to SC4n are enabled to detect an abnormal lamp current.
For example, in the lighting circuit block LC1, if the lighting circuit for the discharge lamp La1 is short circuited, or if the discharge lamp La1 is broken, the lamp current flowing in the lighting circuit for the discharge lamp La1 is caused to increase. With such an increase of the lamp current, the sense voltage inputted to at least one of the comparators CPo1 and CPo2 becomes higher than the normal voltage. When this voltage exceeds the predetermined reference voltage Vth1, a voltage 25b outputted from the comparator CPo1 and/or CPo2 is switched from a low level to a high level.
Also, if a connector open circuit occurs in the lighting circuit block (discharge lamp connector open), or if a lamp open circuit occurs (discharge lamp coming off), the lamp current flowing in the discharge lamp La1 is caused to decrease. With such a decrease of the lamp current, the sense voltage inputted to at least one of the comparators CPs1 and CPs2 becomes lower than the normal voltage. When this voltage comes down below the predetermined reference voltage Vth2, a voltage 25b outputted from the capacitor CPs1 and/or CPs2 is switched from a low level to a high level.
Since the outputs from the comparators CPo1 to CPo4 are OR-connected to the respective outputs from the comparators CPs1 to CPs4 via respective diodes, when a transition to a high level occurs by any one of the outputs from those comparators, the voltage 25b to be inputted to the protection circuit 25 of the control circuit 11 becomes a high level indicating detection of malfunction. When the voltage 25b becomes a high level, the protection circuit 25 outputs a drive stop signal 25a to the logic circuit 24, whereby the logic circuit 24 stops generation of the gate driving signals d1 to d4 thus stopping driving of the bridge circuits BR1 to BRn.
In this connection, in a discharge lamp lighting apparatus for a plurality of discharge lamps, which is used in a backlight for a large LCD television, a number of sense circuits are required corresponding to the number of discharge lamps (for example, the discharge lamp lighting apparatus 100 requires the sense circuits SC1 to SC4n in a number twice as many as that of the discharge lamps La1 to La2n) as described above, and consequently the component and production costs are pushed up, and at the same time a larger mounting space is required thus increasing the apparatus size.
In order to deal with the above problem, an inverter apparatus to drive discharge lamps is disclosed which includes a protection circuit adapted to sense lamp currents flowing in the low voltage side electrodes of the discharge lamps, to synthesize the lamp currents sensed, and to cause the inverter apparatus to cease its operation when the value of the synthesized lamp currents is lower than the reference current value (refer to, for example, Japanese Patent Application Laid-Open No. 2005-317294).
FIG. 6 is a circuit diagram of a conventional inverter apparatus 200 for lighting three discharge lamps as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2005-371294. In the inverter apparatus 200, respective lamp currents in discharge lamps 1a, 1b and 1c are sensed by current sensing resistors R1a, R1b and R1c each disposed between the low voltage side electrode of the discharge lamp 1a/1b/1c and ground such that the lamp currents are converted into voltages. The voltages are rectified by rectification circuits which are respectively composed of resistors R2a, R2b and R2c and diodes D1a, D1b and D1c, and which are disposed in parallel to the current sensing resistors R1a, R1b and R1c. The outputs of the rectification circuits are synthesized by diodes D2a, D2b and D2c into a synthetic feedback to be inputted to the positive terminal (+) of a comparator 3 via a feedback resistor R3. A reference voltage is inputted to the negative terminal (−) of the comparator 3.
The aforementioned reference voltage is set to be lower than the normal value of the synthetic feedback input and higher than the abnormal value thereof, and when the inverter apparatus 200 operates normally, the synthetic feedback input voltage applied to the positive terminal (+) of the comparator 3 is higher than the reference voltage, and the output of the comparator 3 becomes a high level. When the output of the comparator 3 is at a high level, a switch element Q201 is turned on, and the switch element Q201 performs a normal oscillating operation.
When a discharge lamp is broken or is not lit, or when a wire is broken at a lighting circuit, the lamp current is caused to lower thus lowering the synthetic feedback input voltage. And, when the input voltage to the positive terminal (+) of the comparator 3 becomes lower than the reference voltage, the output of the comparator 3 becomes a low level, and the switch element Q201 is turned off, and the switch element Q201 is caused to cease its oscillation.
According to the aforementioned Japanese Patent Application Laid-Open No. 2005-317294, an inverter apparatus can be provided less expensively which features the operation described above, and incorporates a protection circuit favorably comparing in terms of practical performance with circuits previously available.
The inverter apparatus 200 of FIG. 6, however, requires the current sensing resistors R1a to R1c, the rectifier diodes D1a to D1c, and the synthesizer diodes D2a to D2c for a feedback input, thus failing to achieve a significant reduction of components for a sense circuit and rather resulting in increase of the number of components for the sense circuit in proportion to the number of discharge lamps. And, since the sense voltages of the lamp currents are synthesized by the diode D2a to D2c, the sensing accuracy is deteriorated.